cmake touch-ups for Android

Partial merge of glretracer for Android patch set by
Juha-Pekka Heikkila (juha-pekka.heikkila@linux.intel.com)

Aside from the Waffle-based glretracer support in Juha-Pekka's patch set, I wasn't about to build apitrace for Android without these minor cmake changes.

The failure I observed was:

...
-- Check if compiler accepts -pthread
CMake Error: TRY_RUN() invoked in cross-compiling mode, please set the following cache variables appropriately:
   THREADS_PTHREAD_ARG (advanced)
For details see /home/nstewart/dev/apitrace/build/TryRunResults.cmake
-- Check if compiler accepts -pthread - no
...